Integrated switch with internally adjusted conduction time

ABSTRACT

An apparatus and method of providing a pulse width modulated signal that is responsive to a current are disclosed. An integrated circuit according to aspects of the present invention regulates an output of a power supply and includes a switch coupled to receive an external current. The integrated circuit also includes a controller coupled to the switch to control a switching of the external current by the switch in response to an external control signal and an internal current sense signal. The internal current sense signal is proportional to a current in the switch. The output of the power supply is also regulated in the absence of the internal current sense signal.

REFERENCE TO PRIOR APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/543,570, filed Oct. 4, 2006, now pending, entitled“INTEGRATED SWITCH WITH INTERNALLY ADJUSTED CONDUCTION TIME,” which ishereby incorporated by reference.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to power supplies and, morespecifically, the present invention relates to the control of a switchin a switching power supply.

2. Background

In known switching power supplies, a switch is switched on and off toregulate the power that is delivered to an output of the power supplyfrom an input of the power supply in response to a control signal. Thecontrol signal is a feedback signal that regulates the output. There istypically a one-to-one correspondence between the value of the feedbacksignal and the desired conduction time of the switch. Thus, for a givenset of operating conditions, the value of the feedback signal alonedetermines the conduction time of the switch.

In a known power supply where the conduction time of the switch respondsonly to the feedback signal, any disturbance in the operation of thepower supply must propagate through the system until the disturbanceappears in the feedback signal before the switch can respond. Thefeedback circuit is usually slow to respond because it is designed toregulate the average value of an output over many switching periods.Therefore, transient events such as start-up can cause undesirabledeviations in voltage and current within the power supply before thecontrol circuit can establish the desired steady state conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 shows generally an application 100 of one example of anintegrated circuit 105 in accordance with the teachings of the presentinvention.

FIG. 2 is a graph 200 that illustrates generally the behavior of thepulse width modulator 111 according to the example of FIG. 1 inaccordance with the teachings of the present invention.

FIG. 3 illustrates generally one example 300 of an integrated switchincluded in an integrated circuit 105 in accordance with the teaching ofthe invention.

FIG. 4 shows generally waveforms that illustrate the timingrelationships 400 of signals for the example integrated switch 300 ofFIG. 3 in accordance with the teachings of the present invention.

FIG. 5 illustrates generally an example switching power supply inaccordance with the teachings of the present invention.

FIG. 6 shows generally example waveforms of the switched current I_(S1)for three different values of the load when the switch is in position 2to set the input U_(C) to the constant open loop signal U_(COL) inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION

Methods and apparatuses for controlling a switch in a switching powersupply are disclosed. In the following description numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. In other instances, well-knownmaterials or methods have not been described in detail in order to avoidobscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable combinations and/orsubcombinations in one or more embodiments. In addition, it isappreciated that the figures provided herewith are for explanationpurposes to persons ordinarily skilled in the art and that the drawingsare not necessarily drawn to scale.

In various examples of circuits according to the teachings of thepresent invention, the control of a switch in a switching power supplyis described. In one example, the switch and a control circuit areincluded in an integrated circuit. The control circuit receives acontrol signal that corresponds to a desired conduction time of theswitch. In every switching period, the control circuit makes a smallreduction in the desired conduction time in response to the current inthe switch during that switching period. This reduction of conductiontime that is independent of the control signal improves the response ofthe power supply to transient disturbances in accordance with theteachings of the present invention. A reduction in the conduction timeof the switch for an increase in switch current in each switching periodproduces a damping effect that restricts deviations from transientdisturbances, reduces complexity of the feedback circuit, and increasesthe stability of the regulated power supply in accordance with theteachings of the present invention.

In one example, a switch is integrated with a mechanism that is integralwith the switch to make the conduction time of the switch dependent onthe current in the switch in accordance with the teachings of thepresent invention. The dependence of the conduction time on the currentis an inherent property of the integrated switch. No externalconnections or components are required to benefit from the feature. Inone example, the integrated switch senses the current in the switch as avoltage developed across a metal conductor that is part of theintegrated circuit in accordance with the teachings of the presentinvention.

To illustrate, FIG. 1 shows generally an application 100 of one exampleof an integrated circuit 105 in accordance with the teachings of thepresent invention. As shown, an integrated circuit 105 receives acontrol signal U_(C) 110 that may be a voltage or a current. A pulsewidth modulator 111 receives an input signal U_(INPWM) 155 that isrelated to control signal U_(C) 110. The pulse width modulator 111 alsoreceives a periodic timing signal U_(OSC) 125 from an oscillator 120.Timing signal U_(OSC) 125 may be a voltage or a current that has aperiod T_(OSC). Pulse width modulator 111 produces an output U_(OUTPWM)135 that may be a voltage or a current coupled to switch a switch S1130. Switch S1 130 switches an external current I₁ 115 that is coupledto be received by integrated circuit 105. Switch S1 130 conducts for afraction D of period T_(OSC) where D is the duty ratio. The currentI_(S1) 140 in switch S1 130 is sensed as an internal signal U_(S1) 150that may be a voltage or a current that is related to current I_(S1) 140by the factor k_(I) such that

U _(IS) =k _(I) I _(S1)  (1)

In the example, the factor k_(I) has units or dimensions of resistancewhen the internal current sense signal U_(IS) 150 is a voltage. Thefactor k_(I) is dimensionless when the current sense signal U_(IS) 150is a current. The pulse width modulator 111 receives current sensesignal U_(IS) 150 to adjust the duty ratio D in accordance with theteaching of the invention. In one example, it is noted that the currentin the switch, I_(IS), is an analog signal having a continuum of valuesand therefore, the conduction time of the switch as affected by the dutyratio D is responsive to a continuum of values of the current I_(S1) inthe switch in accordance with the teachings of the present invention. Inone example, the continuum of values of the current in the switch,I_(S1), ranges between substantially zero current and a maximumpermissible current in the switch in accordance with the teachings ofthe present invention.

FIG. 2 is a graph 200 that illustrates generally the behavior of thepulse width modulator 111 in the example of FIG. 1. Characteristic linesegments 210 and 220 show the value of duty ratio D of switch S1 130 inresponse to the input signal U_(INPWM) 155 for two values of the currentsense signal U_(IS) 150. Line segments 210 give the value of duty ratioD when the current sense signal U_(IS) 150 is always zero, correspondingto a zero value for the factor k_(I). Line segments 220 give the valueof duty ratio D when the current sense signal U_(IS) 150 is a nonzerofraction of the current I_(S1) 150 in switch S1 130.

FIG. 2 shows that the duty ratio D is between zero and one, increasinglinearly from a minimum value D_(A) to a maximum value D_(B) as theinput signal U_(INPWM) increases from a value U_(A1) or U_(A2) to agreater value U_(B1) or U_(B2), respectively. For an input signalU_(INPWM) with value U_(X) between U_(A2) and U_(B1), the presence of anonzero current sense signal U_(IS) 150 reduces the duty ratio from avalue D_(X) to a lower value D₁. The value of D₁ decreases as the valueof U_(IS) increases.

FIG. 3 illustrates generally one example 300 of an integrated switchincluded in an integrated circuit 105 in accordance with the teaching ofthe invention. Integrated circuit 105 includes a pulse width modulatorcomprising current sources 305 and 315, switches 310 and 320, diode D1317, capacitor C_(D) 330, inverter 345, comparator 350, and AND gate355. An oscillator 120 produces a timing signal U_(OSC) 125 that isperiodic with period T_(OSC). Timing signal U_(OSC) 125 from oscillator120 is low for a fraction k of the period T_(OSC). Timing signal U_(OSC)is coupled to the input of the inverter 345 and to an input of AND gate355. Switch S2 310 is coupled be closed when the output 340 of inverter345 is high. Switch S3 320 is coupled to be closed when timing signalU_(OSC) is high. One skilled in the art will appreciate that currentsources 115, 305, and 315 have finite compliance voltages that causetheir currents to become zero when the respective switches S1 130, S2310, and S3 320 are open.

In one example, switch S1 130 is a metal oxide field effect transistor(MOSFET) coupled to receive a signal U_(OUTPWM) 135 from the output ofAND gate 355 that is also the output of the pulse width modulator.Integrated transistor switch S1 130 switches a current external to theintegrated circuit. In the example, integrated transistor switch S1 130conducts the switched current I_(S1) 380 when the signal U_(OUTPWM) 135is high. A current sense network includes a current sensing element 360,a resistor 370, and a resistor 375. In one example, the current sensingelement 360 is a portion of a layer of metal that is deposited duringthe fabrication of the integrated circuit to make electrical connectionsbetween regions of the integrated circuit. In one example, the currentsensing element 360 has a resistance R_(CS). The switched current I_(S1)380 is sensed as a voltage V_(CS1) 365 on current sensing element 360.As shown, resistors 370 and 375 together form a resistor divider, whichis coupled across current sensing element 360 and scales the voltageV_(CS1) 365 to a current sense voltage nI_(S1) that is the current sensesignal U_(IS) 150.

In the example, integrated circuit 105 receives a control signal U_(C)110 that determines the value of a current I_(FB) 385 from a currentsource 305. Current I_(FB) 385 is the input signal U_(INPWM) 155 of thepulse width modulator 111 shown in FIG. 1. Capacitor C_(D) 330 chargeswith current I_(FB) 385 from current source 305 through switch S2 310while the timing signal U_(OSC) 125 is low. Capacitor C_(D) 330discharges with a substantially fixed current I₀ from current source 315through switch S3 when the timing signal U_(OSC) 125 is high. Diode Dl317 places a lower limit on voltage V_(D) 335. Comparator 350 comparesthe voltage V_(D) 335 on capacitor C_(D) 330 to the internal currentsense signal U_(IS) 150. The output 325 of comparator 350 is high whenthe voltage V_(D) 335 on capacitor C_(D) 335 is greater than the currentsense signal U_(IS) 150. The output 325 of comparator 350 is low whenthe voltage V_(D) 335 on capacitor C_(D) 335 is less than or equal tothe current sense signal U_(IS) 150. Thus, integrated circuit 105switches an integrated switch S1 130 periodically with period T_(OSC)and a conduction time determined jointly by or responsive to both thecontrol signal U_(C) 110 and the switched current I_(S1) 380 inaccordance with the teachings of the present invention.

FIG. 4 shows generally example waveforms that illustrate timingrelationships 400 of signals for the example integrated switch 300 ofFIG. 3. Capacitor C_(D) 330 charges with current I_(FB) 385 from currentsource 305 for a time kT_(OSC) to reach a voltage V_(FB) when timingsignal U_(OSC) 125 goes high. Switch S1 130 then begins to conductcurrent I_(S1) 380. In the example illustrated in FIG. 4, switchedcurrent I_(S1) 380 is a constant current while switch S1 130 isconducting. Capacitor C_(D) 330 then discharges with current lo fromcurrent source 315. Switch S1 130 stops conducting when the voltageV_(D) on capacitor C_(D) reaches the value of current sense signalU_(IS) 150. Thus, conduction time of switch S1 is influenced by thevalue of the current I_(S1) 380 in switch S1 130. For a given value of acontrol signal U_(C) 110, the conduction time of the integrated switchS1 130 is reduced by an amount proportional to the internal currentsense signal U_(IS) 150. Therefore, FIG. 4 shows how the duty ratio D inFIG. 2 is reduced from the value D_(X) to the value D₁ by the influenceof the internal current sense signal U_(IS) 150 in accordance with theteachings of the present invention. Thus in the example, the adjustmentto the duty ratio D, or the conduction time, of the switch in responseto the internal current sense signal U_(IS) 150 is proportional to thecurrent I_(S1) 380 in the switch S1 130 in accordance with the teachingsof the present invention.

FIG. 5 illustrates generally an example of a switching power supply inaccordance with the teachings of the present invention. In theillustrated example, the topology of the power supply in FIG. 5 is knownas a flyback regulator. It is appreciated that there are many topologiesand configurations of switching regulators, and that the flybacktopology shown in FIG. 5 is provided for explanation purposes and thatother power supply topologies may be included in accordance with theteachings of the present invention.

As shown in the illustrated example, the power supply in FIG. 5 providesoutput power to a load 530 from an unregulated input voltage V_(IN) 505.The input voltage V_(IN) 505 is coupled to an energy transfer element T1515 and an integrated switch S1 130. In the example of FIG. 1, it isnoted that the energy transfer element T1 515 may be coupled between aninput of the power supply and an output of the power supply. In theexample of FIG. 5, the energy transfer element T1 515 is illustrated asa transformer with two windings. In general, the transformer can havemore than two windings, with additional windings to provide power toadditional loads, to provide bias voltages, or to sense the voltage at aload. In the example illustrated in FIG. 5, a clamp circuit 510 iscoupled to the primary winding of the energy transfer element T1 525 tocontrol the maximum voltage on the integrated switch S1 130 included inan integrated circuit 570.

In the example, integrated switch S1 130 is switched on and off inresponse to an example controller circuit 575 that includes anoscillator and a pulse width modulator in accordance with the teachingsof the present invention. In one example, switch S1 130 is a transistorsuch as for example a power metal oxide semiconductor field effecttransistor (MOSFET). In operation, example integrated switch S1 130produces pulsating current in the rectifier D1 520 that is filtered bycapacitor C1 525 to produce a substantially constant output voltageV_(O) or a substantially constant output current I_(O) at the load 530.

In one example, the output quantity to be regulated is U_(O) 535, whichin general could be an output voltage V_(O), an output current I_(O), ora combination of the two. In the example, the regulated quantity is notnecessarily fixed, but can be regulated to change in a desired way inresponse to a feedback signal. A feedback circuit 560 is coupled to theoutput quantity U_(O) 535 to produce a feedback signal U_(F) 550 thatmay be an external input U_(C) 110 to the controller 575 that isincluded in the integrated circuit 570. An internal input to thecontroller 545 is the current sense signal U_(IS) 545 that senses aswitched current I_(S1) 540 in switch S1 130 in accordance with theteaching of the present invention.

Switched current I_(S1) in the switching power supply example of FIG. 5is not constant when the switch S1 130 is conducting, but increaseslinearly to a peak value I_(PEAK) during the conduction time that is afraction D of a switching period T_(S). The peak value of the switchedcurrent I_(S1) changes with the current I_(O) in the load 530. In normaloperation, controller 575 changes the duty ratio D of the switch inresponse to feedback signal U_(F) to regulate the output U_(O) 535 inresponse to changes in the input voltage V_(IN) 505 and to changes inthe load 530.

In one example, a switch S4 555 is included in the power supply circuitof FIG. 5 that allows the input U_(C) 110 to be either the feedbacksignal U_(F) 550 or a constant open loop signal U_(COL) 565. In theexample, when the switch S4 555 is in position 1, the input U_(C) to thecontroller 575 is the feedback signal U_(F). When the switch S4 555 isin position 2, the input U_(C) to the controller 575 is the constantopen loop signal U_(COL) 565. In the example, when the switch inposition 2, the influence of the feedback signal is removed, and theduty ratio is responsive only to changes in the peak current I_(PEAK) aswill be shown in FIG. 6 in accordance with the teachings of the presentinvention.

To illustrate, FIG. 6 shows generally example waveforms of the switchedcurrent I_(S1) 540 for three different values of the load 530 when theswitch S4 555 is in position 2 to set the input U_(C) 110 to theconstant open loop signal U_(COL) 565. In the example, duty ratio D_(X)for conduction time D_(X)T_(S) corresponds to a condition when currentsense signal U_(IS) 545 is reduced to zero. As shown, a finite non-zerocurrent sense signal U_(IS) 545 reduces the duty ratio to D₁, D₂, andD₃, corresponding to conduction times D₁T_(S), D₂T_(S), and D₃Ts, forrespective peak currents I_(PEAK LOAD 1), I_(PEAK LOAD2), andI_(PEAK LOAD3), as shown by respective currents 605, 610, and 615. Thus,the internal current sense signal U_(IS) 545 makes adjustments to theconduction time of the internal switch S1 130 such as currents in theinternal integrated switch S1 130 increase, conduction times andcorresponding reduced duty ratios of the switch S1 130 are reduced inaccordance with the teachings of the present invention. Therefore, theadjustment to the conduction time by the current sense signal U_(IS) 545is not sufficient to appreciably affect the regulation of the outputU_(O) 535. In normal operation, with the switch S4 555 in position 1such that the input U_(C) is the feedback signal U_(F) 550, the outputU_(O) 535 of the power supply remains in regulation when the currentsense signal U_(IS) is absent. In other words, the output of the powersupply is regulated even in the absence of the internal current sensesignal in accordance with the teachings of the present invention.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific voltages,currents, frequencies, power range values, times, etc., are provided forexplanation purposes and that other values may also be employed in otherembodiments and examples in accordance with the teachings of the presentinvention.

These modifications can be made to examples of the invention in light ofthe above detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation. The present specification and figures are accordingly tobe regarded as illustrative rather than restrictive.

1. An integrated circuit that regulates an output of a power supply, theintegrated circuit comprising: a controller coupled to control aconduction time of a switch in response to an external control signal,wherein the controller is further coupled to make an adjustment to theconduction time of the switch in response to an internal current sensesignal; and a current sensing element coupled to the controller andcoupled to output the internal current sense signal in response to anexternal current, wherein the current sensing element is a portion of alayer of metal that is deposited during fabrication of the integratedcircuit, wherein a duty ratio of the switch is responsive to theinternal current sense signal.
 2. The integrated circuit of claim 1,wherein the adjustment to the conduction time of the switch isproportional to the internal current sense signal.
 3. The integratedcircuit of claim 1, wherein the conduction time of the switch isresponsive to the external control signal.
 4. The integrated circuit ofclaim 3, wherein the external control signal is representative of afeedback signal that regulates the output of the power supply.
 5. Theintegrated circuit of claim 1, wherein the adjustment to the conductiontime of the switch is a reduction in the conduction time of the switchin response to an increase in the external current.
 6. The integratedcircuit of claim 1, wherein the switch is included in the integratedcircuit.
 7. The integrated circuit of claim 6, wherein the switch iscoupled to receive the external current such that the external currentflows through the switch.
 8. The integrated circuit of claim 1, whereinthe internal current sense signal is proportional to the externalcurrent.